Processing method for manufacturing black tea and an improved black tea

ABSTRACT

Fresh tea leaves or green tea are converted to black tea using tyrosinase oxidation which alters catechines (a group of green tea polyphenols) to theaflavins. Theaflavins are unique black tea polyphenols and a potential anti-sunburn and chemopreventive agents. The oxidation reaction is performed on leaves which contain moisture only in an amount that the oxidation reaction occurs under the conditions of temperature of 20-50° C., air or O 2  (0.2 to 2 moles O 2 /kg dry tea) supply and pH 5.0 to 7.5 at localized sites on and within the tea leaf structure. The moisture is supplied by moistening fresh tea leaves or dry green tea to a limited degree with a proper amount and concentration of monophenol, catechol and polyphenol oxidases, and tyrosinase, or their combinations. Obtained black tea contains much more theaflavins than regular black tea, and is, therefore, a more healthful beverage than that produced through use of conventionally processed black tea.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the processing of tea leaves to produceblack tea.

2. Background Information

The present invention relates to oxidation of green tea leaves to blacktea. As is accepted in the art, green tea is tea which has been freshlypicked and which generally has undergone treatment, such as a heattreatment, to inactivate enzymes contained in the tea which oxidizechemical substances contained in the tea.

Black tea is prepared conventionally by subjecting freshly picked tealeaves to various processing steps which include a fermentation stepwhich employs enzymes naturally present in the tea to effect enzymaticoxidation of chemical substances contained in the tea which results inproviding the organoleptic and aesthetic characteristics, i.e., aroma,flavor and color, associated with aqueous beverage extracts obtainedfrom black tea. Extracts of black tea may be consumed as a hot beverageor may be chilled to provide a cold beverage, or the extracts may beprocessed further to provide an instant water-soluble product forpreparation of hot and cold beverages.

Although beverages prepared from green and Oolong teas are appreciatedby consumers in various parts of the world, in some localities,particularly in the United States, consumer preferences dictate that teabeverages have the organoleptic and aesthetic characteristics ofbeverage extracts obtained from black tea. In contrast to thedistinctive reddish coloration of extracts obtained from black tea,aqueous extracts obtained from green tea, in particular, have ayellow-greenish coloration which tends to reinforce a perception inconsumers that the extracts have a “grassy” flavor and aroma and a“bitter” taste. Oolong teas have organoleptic and aestheticcharacteristics which fall in between those of green and black teas.

Seltzer discloses a process said to enable obtaining fermented black teaand partially fermented tea of more uniform quality from green tea. Toobtain this objective, the process is carried out by extracting greentea leaves with water and then combining the aqueous extract with whatis described as a “relatively small amount” of ‘bruised’ fresh tealeaves and then heating the mixture of the extract and bruised leaves inthe presence of oxygen at a temperature not to exceed 43° C. for aperiod of time after which the reaction mixture is heated to inactivatethe enzymes. The extract obtained is said to have characteristics ofblack tea.

Gurkin discloses treating an aqueous extract of green tea in thepresence of oxygen or treating green tea leaf in the presence of waterand oxygen at a temperature above 50° C., and preferably, at atemperature of from 75° C. to 125° C., under a pressure of at least 100psig (7.03 kg/cm.sup.2), and preferably at a pressure of from 14.06kg/cm.sup.2 to 56.24 kg/cm.sup.2. Times of treatment may range from 2mins to 30 mins. It is taught that, preferably, the pH of the reactionmedia be above a pH of 7 prior to the treatment. In addition todemonstrating the effects of variables of pressure, time, pH andconcentration of tea solids when treating aqueous extracts, Gurkindiscloses treating macerated leaves in water in a ratio of water to leafof 9:1. Gurkin also posits that the treatment may be carried out byadding a “small amount” of water to the green leaf and converting it toblack tea leaf under the disclosed conditions.

Moore, which was assigned commonly with Seltzer and Gurkin, alsodiscloses a process for treating water-soluble constituents of green tealeaves, particularly aqueous extracts thereof, in the manner of Gurkin.Moore, however, differs from Gurkin in that the process requires thatthe reaction media have a pH of at least 7.5. It is disclosed that sucha pH was found to be a “major” factor affecting the color of the finalproduct and that such a pH is necessary to produce a “practical degreeof conversion within a commercially feasible time”. When leaves are tobe treated, Moore teaches that they are to be treated in an alkalinesolution in which the majority of the solution is absorbed by the leaf.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved blacktea manufacturing process through the use of the product of whichproduces a beverage of enhanced therapeutic or health enhancementqualities.

It is an object of the present invention to provide an improved blacktea manufacturing process through the use of the product of whichproduces a beverage which elevated, beneficial theaflavins, whencompared with beverages made from conventionally prepared black tea.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Mushroom tyrosinase rapidly reacts with dioxygen and then release oxygenand become deoxytyrosinase by oxidation of phenols or polyphenols. Infact tyrosinase is a catalyst in oxidation of phenols.

The present invention is characterized in that polyphenolic substancescontained in moist fresh tea leaves having a moisture content of atleast 20% by weight based upon the dry weight of tea solids contained inthe moist leaves (“by weight dry tea solids”) are oxidized at about20-50° C. by spraying tyrosinase solution on them. For dry green tea,tyrosinase solution is used to soak the leaves. The oxidation durationis 10 to 360 minutes at a good air flow environment or supplying oxygen.

The reaction is characterized further in that the moist leaves arecontacted with tyrosinase and molecular oxygen sufficient to the moistleaves to oxidize polyphenolic substances contained in the moist leaves.The treated leaves may be processed immediately for drying to obtainblack tea, or preparing water-soluble instant tea products, or they maybe dried for subsequent extraction for preparation of a beverage.

It has been discovered that the amount of moisture employed in theoxidizing treatment of the present invention is a very criticalvariable. By reason of the amount of moisture employed in the process ofthe present invention, the leaves to be treated are only moist which isbelieved to facilitate the permeation of the leaves by the oxidizingagent. Thus, in the process of the present invention, by reason of theleaves being only moist and thereby having almost no free water present,the oxidation reaction takes place in localized sites on and within thetea leaf structure. Therefore, the pH changes which occur due to theoxidation reaction occur substantially only at the localized sites anddo not substantially affect neighboring oxidation reaction sites, whichwould occur if free water were present.

In carrying out the process of the present invention, the moist tealeaves, and hence the various oxidizable substances of the tea leaves,are contacted with molecular oxygen. Although various oxidizing agentscan be employed to provide the molecular oxygen for the reaction,various chemical agents such as hydrogen peroxide or permanganate, forexample, may be deemed undesirable because residues of the same in thetreated leaves may be considered to be food additives, and thus, thefinal product therefore would not be considered to be 100% tea. Thus,gaseous oxidizing agents are most preferred.

Suitable gaseous oxidizing agents include ozone or ozone-containinggases, but more advantageously, an oxygen-containing gas, including airand oxygen enriched air may be employed. For most efficient results,however, oxygen gas is employed as the oxidizing agent.

When the tea leaves to be treated in accordance with the presentinvention are in a dry state, having a stable moisture content whichconventionally is in a range of from about 5% to about 7% by weight drytea solids, the leaves first are moistened with water solution oftyrosinase, conveniently in the vessel in which the oxidation step is tobe performed. The moistening step should produce moisture contents nogreater than approximately 40% by weight of dry tea solids. Higherlevels of moisture should be avoided since such levels generally willresult in saturation of the leaves and the presence of free water, thelatter adversely affecting the quality of the oxidation reaction asdiscussed elsewhere herein.

The object of the moistening step is to moisten the leaves uniformly,and to that end, preferably, the leaves are moistened by spraying themwith water by means such as with a series of nozzles contained in thevessel. Preferably, the leaves being moistened are agitated, such aswith a stirrer, by tumbling, or by a fluidized bed, or other suchagitating means. It would be preferred that the moist leaves andmolecular oxygen be contacted in a countercurrent flow. For bestresults, after adding water solution of enzymes to the leaves, themoisture is allowed to equilibrate throughout the leaves, preferablywhile agitating the leaves, so that the moisture is substantiallyuniformly imbibed by and distributed in the leaves and so that there isno free water between and amongst the moist leaves to be treated in theoxidizing step.

EXAMPLES

The following examples are illustrative of the present invention andparts and percentages are by dry weight unless otherwise indicated.Theaflavins were quantified by HPLC and using standard theaflavins fromSIGMA Company. Tyrosinase from SIGMA is composed of monophenol oxidase,polyphenol oxidase, catechol oxidase and oxidoreductase. It should benoted that these examples are only that —examples —a wide range ofconditions, such as time and units of enzyme, can be used because moretime in process will offset a lesser enzyme presence, and vice versa.

Example I

About 0.35 kg of a sample from a batch of dry Chinese green tea leavesare moistened in a vessel to a moisture content of about 25% by weightdry tea solids by spraying them with mushroom tyrosinase (SIGMA, 500,000units) solution (pH 6.5) while agitating them. After adding thetyrosinase, agitation is continued for enabling the moisture toequilibrate throughout the leaves under the room temperature for 60minutes. While agitating the moist leaves in the vessel, air isintroduced into the vessel. Upon finishing the reaction, tea leaves weredried to get new black tea I.

An infusion extract obtained from the treated leaves for preparing a0.1% by weight extract. It is found that the new black tea I contains 9%of theaflavins which are much higher than average black tea with about1-4% of theaflavins.

Example II

About 0.5 kg of a sample from a batch of dry Chinese green tea leavesare moistened in a vessel to a moisture content of about 30% by weightdry tea solids by spraying them with mushroom tyrosinase (SIGMA, 500,000units) solution (pH 6.5) while agitating them. After adding thetyrosinase, agitation is continued for enabling the moisture toequilibrate throughout the leaves under the room temperature for 90minutes. While agitating the moist leaves in the vessel, air isintroduced into the vessel.

Upon finishing the reaction, tea leaves were dried to get new black teaII. An infusion extract obtained from the treated leaves for preparing a0.1% by weight extract. It is found that the new black tea II contains12% of theaflavins.

Example III

About 0.2 kg of a sample from a batch of dry Chinese green tea leavesare moistened in a vessel to a moisture content of about 25% by weightdry tea solids by spraying them with mushroom tyrosinase (SIGMA, 250,000units) solution (pH 5.5) while agitating them. After adding thetyrosinase, agitation is continued for enabling the moisture toequilibrate throughout the leaves under the room temperature for 30minutes. While agitating the moist leaves in the vessel, air isintroduced into the vessel. Upon finishing the reaction, tea leaves weredried to get new black tea III.

An infusion extract obtained from the treated leaves for preparing a0.1% by weight extract. It is found that the new black tea III contains8% of theaflavins.

In view of the foregoing, it is clear that practice of the presentinvention will yield black tea, the use of which will yield a beverageof higher, beneficial polyphenol content. This, in turn, represents a“painless” source of chemorepressive and sunburn resistance substancesfor consumption by consumers.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitedsense. Various modifications of the disclosed embodiments, as well asalternative embodiments of the invention will become apparent to personsskilled in the art upon the reference to the description of theinvention. It is, therefore, contemplated that the appended claims willcover such modifications that fall within the scope of the invention.

We claim:
 1. An improved black tea produced through a process involvingthe steps of: selecting a measure of tea leaves; moistening surfaces ofsaid tea leaves with an aqueous tyrosinase solution without immersingsaid tea leaves therein and only to a degree that substantially no freeliquid of said aqueous tyrosinase solution remains after saidmoistening; exposing said tea leaves, once moistened, to an oxidizingagent; drying said tea leaves to derive black tea.
 2. The black tea ofclaim 1 wherein the moisture content of said tea leaves after saidmoistening is no greater than approximately 40% of the weight of saidtea leaves prior to said moistening.
 3. The black tea of claim 1 whereinsaid oxidizing agent is an oxygen-containing gas.
 4. The black tea ofclaim 2 wherein said oxidizing agent is an oxygen-containing gas.
 5. Theblack tea of claim 1 wherein said exposing of said tea leaves, oncemoistened, to an oxidizing agent, occurs at between approximately 20° C.and 50° C.
 6. The black tea of claim 2 wherein said exposing of said tealeaves, once moistened, to an oxidizing agent, occurs at betweenapproximately 20° C. and 50° C.